Why do some men, act apathetic? What's up with that?

Coming from an apathetic male, i would say its not acting, or he has truly been damaged by someone close to him in the past and finds it to painful to really care. In either case, if your still really not over him, i would ask him that exact question, but not in a demanding or intrusive way (because if it is the second case he will just "shell up"). If its the first case, I'm sorry but you truly are better off without him, especially if hes not meeting your needs of affection.